Helium-3

From Wikipedia, the free encyclopedia

Helium-3
Helium-3 Full table
General
Name, symbol	Helium-3, He-3,3He
Neutrons	1
Protons	2
Nuclide data
Natural abundance	0.000137% (% He on Earth)
Half-life	stable
Parent isotopes	3H (beta decay of tritium)
Isotope mass	3.0160293 u
Spin	1⁄2

Helium-3 (He-3) is a light, non-radioactive isotope of helium with two protons and one neutron, in contrast with two neutrons in common helium. Its hypothetical existence was first proposed in 1934 by the Australian nuclear physicist Mark Oliphant while he was working at the University of Cambridge Cavendish Laboratory. Oliphant had performed experiments in which fast deuterons collided with deuteron targets (incidentally, the first demonstration of nuclear fusion).^[1] Helium-3 was thought to be a radioactive isotope until helions were also found in samples of natural helium, which is mostly helium-4, taken both from the terrestrial atmosphere and from natural gas wells.^[2]

Helium-3 occurs as a primordial nuclide, escaping from the Earth's crust into the atmosphere and into outer space over millions of years. Helium-3 is also thought to be a natural nucleogenic and cosmogenic nuclide, one produced when lithium is bombarded by natural neutrons. Those are released by spontaneous fission and by nuclear reactions with cosmic rays. Some of the helium-3 found in the terrestrial atmosphere is also a relic of atmospheric and underwater nuclear weapons testing, conducted by the three big nuclear powers before 1963. Most of this comes from the decay of tritium (hydrogen-3), which decays into helium-3 with a half life of 12.3 years. Furthermore, some nuclear reactors (landbound or shipbound) periodically release some helium-3 and tritium into the atmosphere. The nuclear reactor disaster at Chernobyl released a huge amount of radioactive tritium into the atmosphere, and smaller accidents have caused smaller releases. Furthermore, significant amounts of tritium and helium-3 have been deliberately produced in national arsenal nuclear reactors by the irradiation of lithium-6. The tritium is used to "boost" nuclear weapons, and some of this inevitably escapes during its production, transportation, and storage. Hence, helium-3 enters the atmosphere both through its direct release and through the radioactive decay of tritium.

The abundance of helium-3 is thought to be greater on the Moon than on Earth, having been embedded in the upper layer of regolith by the solar wind over billions of years,^[3] though still lower in quantity than in the solar system's gas giants.^[4][5]

Physical properties

Because of its lower atomic mass of 3.02 atomic mass units, helium-3 has some physical properties different from those of helium-4, with a mass of 4.00 atomic mass units. Because of the weak, induced dipole–dipole interaction between helium atoms, their macroscopic physical properties are mainly determined by their zero-point energy. Also, the microscopic properties of helium-3 cause it to have a higher zero-point energy than helium-4. This implies that helium-3 can overcome dipole–dipole interactions with less thermal energy than helium-4 can.

The quantum mechanical effects on helium-3 and helium-4 are significantly different because with two protons, two neutrons, and two electrons, helium-4 has an overall spin of zero, making it a boson, but with one fewer neutron, helium-3 has an overall spin of one half, making it a fermion.

Helium-3 boils at 3.19 K compared with helium-4 at 4.23 K, and its critical point is also lower at 3.35 K, compared with helium-4 at 5.2 K. Helium-3 has less than one-half of the density when it is at its boiling point: 59 gram per liter compared to the 125 gram per liter of helium-4—at a pressure of one atmosphere. Its latent heat of vaporization is also considerably lower at 0.026 kilojoule per mole compared with the 0.0829 kilojoule per mole of helium-4.^[6]

Fusion reactions

Comparison of neutronicity of reactions^{[7][8][9][10][11]}

Reactants		Products	Q	n/MeV
First-generation fusion fuels
21H + 21H (D-D)	→	32He + 10n	3.268 MeV	0.306
21H + 21H (D-D)	→	31H + 11p	4.032 MeV	0
21H + 31H (D-T)	→	42He + 10n	17.571 MeV	0.057
Second-generation fusion fuel
21H + 32He (D-3He)	→	42He + 11p	18.354 MeV	0
Third-generation fusion fuels
32He + 32He	→	42He+ 211p	12.86 MeV	0
115B + 11p	→	3 42He	8.68 MeV	0
Net result of D burning (sum of first 4 rows)
6D	→	2(4He + n + p)	43.225 MeV	0.046
Current nuclear fuel
235U + n	→	2 FP+ 2.5n	~200 MeV	0.001

The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The DT rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.

³He can be used in fusion reactions by either of the reactions ²D + ³He →   ⁴He +  ¹p + 18.3 MeV, or ³He + ³He → ⁴He   + 2 ¹p+ 12.86 MeV

The conventional deuterium + tritium ("D-T") fusion process produces energetic neutrons which render reactor components radioactive with activation products. The appeal of helium-3 fusion stems from the aneutronic nature of its reaction products. Helium-3 itself is non-radioactive. The lone high-energy by-product, the proton, can be contained using electric and magnetic fields. The momentum energy of this proton (created in the fusion process) will interact with the containing electromagnetic field, resulting in direct net electricity generation.^[12]

Because of the higher Coulomb barrier, the temperatures required for ²₁H + ³₂He fusion are much higher than those of conventional D-T fusion. Moreover, since both reactants need to be mixed together to fuse, reactions between nuclei of the same reactant will occur, and the D-D reaction (²₁H + ²₁H) does produce a neutron. Reaction rates vary with temperature, but the D-³He reaction rate is never greater than 3.56 times the D-D reaction rate (see graph). Therefore fusion using D-³He fuel may produce a somewhat lower neutron flux than D-T fusion, but is by no means clean, negating some of its main attraction.

The second possibility, fusing ³₂He with itself (³₂He + ³₂He), requires even higher temperatures (since now both reactants have a +2 charge), and thus is even more difficult than the D-³He reaction.
However, it does offer a possible reaction that produces no neutrons; the protons it produces possess charges and can be contained using electric and magnetic fields, which in turn results in direct electricity generation. ³₂He + ³₂He fusion has been demonstrated in the laboratory and is thus theoretically feasible and would have immense advantages, but commercial viability is many years in the future.^[13]

The amounts of helium-3 needed as a replacement for conventional fuels are substantial by comparison to amounts currently available. The total amount of energy produced in the ²₁H + ³₂He reaction is 18.4 MeV, which corresponds to some 493 megawatt-hours (4.93×10⁸ W·h) per three grams (one mole) of ³He. If the total amount of energy could be converted to electrical power with 100% efficiency (a physical impossibility), it would correspond to about 30 minutes of output of a gigawatt electrical plant per mole of ³He. Thus, a year's production (at 6 grams for each operation hour) would require 52.5 kilograms of helium-3.^{[citation needed]} The amount of fuel needed for large-scale applications can also be put in terms of total consumption: electricity consumption by 107 million U.S. households in 2001^[14] totaled 1,140 billion kW·h (1.14×10¹⁵ W·h). Again assuming 100% conversion efficiency, 6.7 tonnes per year of helium-3 would be required for that segment of the energy demand of the United States, 15 to 20 tonnes per year given a more realistic end-to-end conversion efficiency.^{[citation needed]}

Neutron detection

Helium-3 is a most important isotope in instrumentation for neutron detection. It has a high absorption cross section for thermal neutron beams and is used as a converter gas in neutron detectors. The neutron is converted through the nuclear reaction

n + ³He → ³H + ¹H + 0.764 MeV

into charged particles tritium (T, ³H) and protium (p, ¹H) which then are detected by creating a charge cloud in the stopping gas of a proportional counter or a Geiger-Müller tube.^[15]

Furthermore, the absorption process is strongly spin-dependent, which allows a spin-polarized helium-3 volume to transmit neutrons with one spin component while absorbing the other. This effect is employed in neutron polarization analysis, a technique which probes for magnetic properties of matter.^{[16][17][18][19]}

The United States Department of Homeland Security had hoped to deploy detectors to spot smuggled plutonium in shipping containers by their neutron emissions, but the worldwide shortage of helium-3 following the drawdown in nuclear weapons production since the Cold War has to some extent prevented this.^[20] As of 2012, DHS determined the commercial supply of boron-10 would support converting its neutron detection infrastructure to that technology.^[21]

Cryogenics

A helium-3 refrigerator uses helium-3 to achieve temperatures of 0.2 to 0.3 kelvin. A dilution refrigerator uses a mixture of helium-3 and helium-4 to reach cryogenic temperatures as low as a few thousandths of a kelvin.^[22]

An important property of helium-3, which distinguishes it from the more common helium-4, is that its nucleus is a fermion since it contains an odd number of spin ¹⁄₂ particles. Helium-4 nuclei are bosons, containing an even number of spin ¹⁄₂ particles. This is a direct result of the addition rules for quantized angular momentum. At low temperatures (about 2.17 K), helium-4 undergoes a phase transition: A fraction of it enters a superfluid phase that can be roughly understood as a type of Bose–Einstein condensate. Such a mechanism is not available for helium-3 atoms, which are fermions. However, it was widely speculated that helium-3 could also become a superfluid at much lower temperatures, if the atoms formed into pairs analogous to Cooper pairs in the BCS theory of superconductivity. Each Cooper pair, having integer spin, can be thought of as a boson. During the 1970s, David Lee, Douglas Osheroff and Robert Coleman Richardson discovered two phase transitions along the melting curve, which were soon realized to be the two superfluid phases of helium-3.^[23][24] The transition to a superfluid occurs at 2.491 millikelvins (i.e., 0.002491 K) on the melting curve. They were awarded the 1996 Nobel Prize in Physics for their discovery. Tony Leggett won the 2003 Nobel Prize in Physics for his work on refining understanding of the superfluid phase of helium-3.^[25]

In zero magnetic field, there are two distinct superfluid phases of ³He, the A-phase and the B-phase. The B-phase is the low-temperature, low-pressure phase which has an isotropic energy gap. The A-phase is the higher temperature, higher pressure phase that is further stabilized by a magnetic field and has two point nodes in its gap. The presence of two phases is a clear indication that ³He is an unconventional superfluid (superconductor), since the presence of two phases requires an additional symmetry, other than gauge symmetry, to be broken. In fact, it is a p-wave superfluid, with spin one, S=1, and angular momentum one, L=1. The ground state corresponds to total angular momentum zero, J=S+L=0 (vector addition). Excited states are possible with non-zero total angular momentum, J>0, which are excited pair collective modes. Because of the extreme purity of superfluid ³He (since all materials except ⁴He have solidified and sunk to the bottom of the liquid ³He and any ⁴He has phase separated entirely, this is the most pure condensed matter state), these collective modes have been studied with much greater precision than in any other unconventional pairing system.

Medical lung imaging

Helium-3 nuclei have an intrinsic nuclear spin of ¹⁄₂, and a relatively high magnetogyric ratio. Helium-3 can be hyperpolarized using non-equilibrium means such as spin-exchange optical pumping.^[26] During this process, circularly polarized infrared laser light, tuned to the appropriate wavelength, is used to excite electrons in an alkali metal, such as caesium or rubidium inside a sealed glass vessel. The angular momentum is transferred from the alkali metal electrons to the noble gas nuclei through collisions. In essence, this process effectively aligns the nuclear spins with the magnetic field in order to enhance the NMR signal. The hyperpolarized gas may then be stored at pressures of 10 atm, for up to 100 hours. Following inhalation, gas mixtures containing the hyperpolarized helium-3 gas can be imaged with an MRI scanner to produce anatomical and functional images of lung ventilation. This technique is also able to produce images of the airway tree, locate unventilated defects, measure the alveolar oxygen partial pressure, and measure the ventilation/perfusion ratio. This technique may be critical for the diagnosis and treatment management of chronic respiratory diseases such as chronic obstructive pulmonary disease (COPD), emphysema, cystic fibrosis, and asthma.^[27]

Production

Current US industrial consumption of helium-3 is approximately 60,000 liters (approximately 8 kg) per year;^[28] cost at auction has typically been approximately $100/liter although increasing demand has raised prices to as much as $2,000/liter in recent years.^[29] Helium-3 is naturally present in small quantities due to radioactive decay, but virtually all helium-3 used in industry is manufactured. Helium-3 is a product of tritium decay, and tritium can be produced through neutron bombardment of deuterium, lithium, boron, or nitrogen targets. Production of tritium in significant quantities requires the high neutron flux of a nuclear reactor; breeding tritium with lithium-6 consumes the neutron, while breeding with lithium-7 produces a low energy neutron as a replacement for the consumed fast neutron.

Current supplies of helium-3 come, in part, from the dismantling of nuclear weapons where it accumulates,^[30][31] however the need for warhead disassembly is diminishing. Consequently tritium itself is in short supply, and the US Department of Energy recently began producing it by the lithium irradiation method at the Tennessee Valley Authority's Watts Bar reactor.^[28] Substantial quantities of tritium could also be extracted from the heavy water moderator in CANDU nuclear reactors.

Production of helium-3 from tritium at a rate sufficient to meet world demand will require significant investment, as tritium must be produced at the same rate as helium-3, and approximately eighteen times as much tritium must be maintained in storage as the amount of helium-3 produced annually by decay (production rate ^dN⁄_dt from number of moles or other unit mass of tritium N, is N γ = N ^{(ln 2)}⁄_t1/2 where the value of ^t_1/2⁄_{(ln 2)} is about 18 years; see radioactive decay). If commercial fusion reactors were to use helium-3 as a fuel, they would require tens of tonnes of helium-3 each year to produce a fraction of the world's power, requiring substantial expansion of facilities for tritium production and storage.^[32]

Abundance

Solar nebula (primordial) abundance

One early estimate of the primordial ratio of ³He to ⁴He in the solar nebula has been the measurement of their ratio in the atmosphere of Jupiter, measured by the mass spectrometer of the Galileo atmospheric entry probe. This ratio is about 1:10,000,^[33] or 100 parts of ³He per million parts of ⁴He. This is roughly the same ratio of the isotopes in lunar regolith, when it contains 28 ppm helium-4 and 2.8 ppb helium-3 (which is at the lower end of actual sample measurements, which vary from about 1.4 to 15 ppb). However, terrestrial ratios of the isotopes are lower by a factor of 100, mainly due to enrichment of helium-4 stocks in the mantle by billions of years of alpha decay from uranium and thorium.

Terrestrial abundance

³He is a primordial substance in the Earth's mantle, considered to have become entrapped within the Earth during planetary formation. The ratio of ³He to ⁴He within the Earth's crust and mantle is less than that for assumptions of solar disk composition as obtained from meteorite and lunar samples, with terrestrial materials generally containing lower ³He/⁴He ratios due to ingrowth of ⁴He from radioactive decay.
In the space, ³He has a ratio of 300 atoms per million atoms of ⁴He (at. ppm),^[34] the original ratio of these primodal gases in mantle was around 200-300 ppm when Earth was formed. A lot of ⁴He was generated by alpha-particle decay of uranium and thorium, and now mantle has only around 7% primodal helium,^[35] lowering the total ³He/⁴He ratio to around 20 at ppm. Ratios of ³He/⁴He in excess of atmospheric are indicative of a contribution of ³He from the mantle. Crustal sources are dominated by the ⁴He which is produced by the decay of radioactive elements in the crust and mantle.

The ratio of helium-3 to helium-4 in natural Earth-bound sources varies greatly.^[36][37] Samples of the lithium ore spodumene from Edison Mine, South Dakota were found to contain 12 parts of helium-3 to a million parts of helium-4. Samples from other mines showed 2 parts per million.^[36]

Helium is also present as up to 7% of some natural gas sources,^[38] and large sources have over 0.5% (above 0.2% makes it viable to extract).^[39] Algeria's annual gas production is assumed to contain 100 million normal cubic metres^[39] and this would contain between 5 and 50 m³ of helium-3 (about 1 to 10 kilograms) using the normal abundance range of 0.5 to 5 ppm. Similarly the US 2002 stockpile of 1 billion normal m^3[39] would have contained about 10 to 100 kilograms of helium-3.

³He is also present in the Earth's atmosphere. The natural abundance of ³He in naturally occurring helium gas is 1.38×10⁻⁶ (1.38 parts per million). The partial pressure of helium in the Earth's atmosphere is about 0.52 Pa, and thus helium accounts for 5.2 parts per million of the total pressure (101325 Pa) in the Earth's atmosphere, and ³He thus accounts for 7.2 parts per trillion of the atmosphere. Since the atmosphere of the Earth has a mass of about 5.14×10¹⁵ tonnes,^[40] the mass of ³He in the Earth's atmosphere is the product of these numbers, or about 37,000 tonnes of ³He.

³He is produced on Earth from three sources: lithium spallation, cosmic rays, and beta decay of tritium (³H). The contribution from cosmic rays is negligible within all except the oldest regolith materials, and lithium spallation reactions are a lesser contributor than the production of ⁴He by alpha particle emissions.

The total amount of helium-3 in the mantle may be in the range of 0.1–1 million tonnes. However, most of the mantle is not directly accessible. Some helium-3 leaks up through deep-sourced hotspot volcanoes such as those of the Hawaiian Islands, but only 300 grams per year is emitted to the atmosphere. Mid-ocean ridges emit another 3 kilogram per year. Around subduction zones, various sources produce helium-3 in natural gas deposits which possibly contain a thousand tonnes of helium-3 (although there may be 25 thousand tonnes if all ancient subduction zones have such deposits). Wittenberg estimated that United States crustal natural gas sources may have only half a tonne total.^[41] Wittenberg cited Anderson's estimate of another 1200 metric tonnes in interplanetary dust particles on the ocean floors.^[42] In the 1994 study, extracting helium-3 from these sources consumes more energy than fusion would release.^[43] Wittenberg also writes that extraction from US crustal natural gas, consumes ten times the energy available from fusion reactions.^{[44][clarification needed]}

Extraterrestrial abundance

Materials on the Moon's surface contain helium-3 at concentrations on the order of between 1.4 and 15 ppb in sunlit areas,^[45][46] and may contain concentrations as much as 50 ppb in permanently shadowed regions.^[5] A number of people, starting with Gerald Kulcinski in 1986,^[47] have proposed to explore the moon, mine lunar regolith and use the helium-3 for fusion. Because of the low concentrations of helium-3, any mining equipment would need to process extremely large amounts of regolith (over 150 million tonnes of regolith to obtain one ton of helium 3),^[48] and some proposals have suggested that helium-3 extraction be piggybacked onto a larger mining and development operation.^{[citation needed]}

The primary objective of Indian Space Research Organization's first lunar probe called Chandrayaan-I, launched on October 22, 2008, was reported in some sources to be mapping the Moon's surface for helium-3-containing minerals.^[49] However, this is debatable; no such objective is mentioned in the project's official list of goals, while at the same time, many of its scientific payloads have noted helium-3-related applications.^[50][51]

Cosmochemist and geochemist Ouyang Ziyuan from the Chinese Academy of Sciences who is now in charge of the Chinese Lunar Exploration Program has already stated on many occasions that one of the main goals of the program would be the mining of helium-3, from which operation "each year three space shuttle missions could bring enough fuel for all human beings across the world."^[52] To "bring enough fuel for all human beings across the world",^[32] more than one Space Shuttle load (and the processing of 4 million tonnes of regolith) per week, at least 52 per year, would be necessary.^{[citation needed][dubious – discuss]}

In January 2006, the Russian space company RKK Energiya announced that it considers lunar helium-3 a potential economic resource to be mined by 2020,^[53] if funding can be found.^[54][55]

Mining gas giants for helium-3 has also been proposed.^[56] The British Interplanetary Society's hypothetical Project Daedalus interstellar probe design was fueled by helium-3 mines in the atmosphere of Jupiter, for example. Jupiter's high gravity makes this a less energetically favorable operation than extracting helium-3 from the other gas giants of the solar system, however.

Power generation

A second-generation approach to controlled fusion power involves combining helium-3 (³₂He) and deuterium (²₁H). This reaction produces a helium-4 ion (⁴₂He) (like an alpha particle, but of different origin) and a high-energy proton (positively charged hydrogen ion) (¹₁p). The most important potential advantage of this fusion reaction for power production as well as other applications lies in its compatibility with the use of electrostatic fields to control fuel ions and the fusion protons. Protons, as positively charged particles, can be converted directly into electricity, through use of solid-state conversion materials as well as other techniques. Potential conversion efficiencies of 70% may be possible, as there is no need to convert proton energy to heat in order to drive a turbine-powered electrical generator^{[citation needed]}.

There have been many claims about the capabilities of helium-3 power plants. According to proponents, fusion power plants operating on deuterium and helium-3 would offer lower capital and operating costs than their competitors due to less technical complexity, higher conversion efficiency, smaller size, the absence of radioactive fuel, no air or water pollution, and only low-level radioactive waste disposal requirements. Recent estimates suggest that about $6 billion in investment capital will be required to develop and construct the first helium-3 fusion power plant. Financial breakeven at today's wholesale electricity prices (5 US cents per kilowatt-hour) would occur after five 1-gigawatt plants were on line, replacing old conventional plants or meeting new demand.^[57]

The reality is not so clear-cut. The most advanced fusion programs in the world are inertial confinement fusion (such as National Ignition Facility) and magnetic confinement fusion (such as ITER and other tokamaks). In the case of the former, there is no solid roadmap to power generation. In the case of the latter, commercial power generation is not expected until around 2050.^[58] In both cases, the type of fusion discussed is the simplest: D-T fusion. The reason for this is the very low Coulomb barrier for this reaction; for D+³He, the barrier is much higher, and it is even higher for ³He–³He. The immense cost of reactors like ITER and National Ignition Facility are largely due to their immense size, yet to scale up to higher plasma temperatures would require reactors far larger still. The 14.7 MeV proton and 3.6 MeV alpha particle from D–³He fusion, plus the higher conversion efficiency, means that more electricity is obtained per kilogram than with D-T fusion (17.6 MeV), but not that much more. As a further downside, the rates of reaction for helium-3 fusion reactions are not particularly high, requiring a reactor that is larger still or more reactors to produce the same amount of electricity.

To attempt to work around this problem of massively large power plants that may not even be economical with D-T fusion, let alone the far more challenging D–³He fusion, a number of other reactors have been proposed – the Fusor, Polywell, Focus fusion, and many more, though many of these concepts have fundamental problems with achieving a net energy gain, and generally attempt to achieve fusion in thermal disequilibrium, something that could potentially prove impossible,^[59] and consequently, these long-shot programs tend to have trouble garnering funding despite their low budgets. Unlike the "big", "hot" fusion systems, however, if such systems were to work, they could scale to the higher barrier "aneutronic" fuels, and therefore their proponents tend to promote p-B fusion, which requires no exotic fuels like helium-3.

Solar sail

From Wikipedia, the free encyclopedia

IKAROS spaceprobe with solar sail in flight (artist's depiction) showing a typical square sail configuration

Solar sails (also called light sails or photon sails) are a form of spacecraft propulsion using the radiation pressure (also called solar pressure) from stars to push large ultra-thin mirrors to high speeds. Light sails could also be driven by energy beams to extend their range of operations, which is strictly beam sailing rather than solar sailing.
Solar sail craft offer the possibility of low-cost operations combined with long operating lifetimes.
Since they have few moving parts and use no propellant, they can potentially be used numerous times for delivery of payloads.

Solar sails use a phenomenon that has a proven, measured effect on spacecraft. Solar pressure affects all spacecraft, whether in interplanetary space or in orbit around a planet or small body. A typical spacecraft going to Mars, for example, will be displaced by thousands of kilometres by solar pressure, so the effects must be accounted for in trajectory planning, which has been done since the time of the earliest interplanetary spacecraft of the 1960s. Solar pressure also affects the attitude of a craft, a factor that must be included in spacecraft design.^[1]

The total force exerted on an 800 by 800 meter solar sail, for example, is about 5 newtons (1.1 lbf) at Earth's distance from the Sun,^[2] making it a low-thrust propulsion system, similar to spacecraft propelled by electric engines.

History of concept

Johannes Kepler observed that comet tails point away from the Sun and suggested that the Sun caused the effect. In a letter to Galileo in 1610, he wrote, "Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void." He might have had the comet tail phenomenon in mind when he wrote those words, although his publications on comet tails came several years later.^[3]

James Clerk Maxwell, in 1861–64, published his theory of electromagnetic fields and radiation, which shows that light has momentum and thus can exert pressure on objects. Maxwell's equations provide the theoretical foundation for sailing with light pressure. So by 1864, the physics community and beyond knew sunlight carried momentum that would exert a pressure on objects.
Jules Verne, in From the Earth to the Moon, published in 1865, wrote "there will some day appear velocities far greater than these [of the planets and the projectile], of which light or electricity will probably be the mechanical agent ... we shall one day travel to the moon, the planets, and the stars." This is possibly the first published recognition that light could move ships through space. Given the date of his publication and the widespread, permanent distribution of his work, it appears that he should be regarded as the originator of the concept of space sailing by light pressure, although he did not develop the concept further^{[original research?]}. Verne probably got the idea directly and immediately from Maxwell's 1864 theory (although it cannot be ruled out that Maxwell or an intermediary recognized the sailing potential and became the source for Verne).^[4]

Pyotr Lebedev was first to successfully demonstrate light pressure, which he did in 1899 with a torsional balance;^[5] Ernest Nichols and Gordon Hull conducted a similar independent experiment in 1901 using a Nichols radiometer.^[6]

Albert Einstein provided a different formalism by his recognizing the equivalence of mass and energy. He simply wrote p = E/c as the relationship between the momentum, the energy, and the speed of light.

Svante Arrhenius predicted in 1908 the possibility of solar radiation pressure distributing life spores across interstellar distances, the concept of panspermia. He apparently was the first scientist to state that light could move objects between stars.^[7]

Friedrich Zander (Tsander) published a technical paper in 1925 that included technical analysis of solar sailing. Zander wrote of "using tremendous mirrors of very thin sheets" and "using the pressure of sunlight to attain cosmic velocities".^[8]

JBS Haldane speculated in 1927 about the invention of tubular spaceships that would take humanity to space and how "wings of metallic foil of a square kilometre or more in area are spread out to catch the Sun's radiation pressure".^[9]

J.D. Bernal wrote in 1929, "A form of space sailing might be developed which used the repulsive effect of the Sun's rays instead of wind. A space vessel spreading its large, metallic wings, acres in extent, to the full, might be blown to the limit of Neptune's orbit. Then, to increase its speed, it would tack, close-hauled, down the gravitational field, spreading full sail again as it rushed past the Sun."^[10]

The first formal technology and design effort for a solar sail began in 1976 at Jet Propulsion Laboratory for a proposed mission to rendezvous with Halley's Comet.^[2]

Physical principles

Solar radiation pressure

Solar radiation exerts a pressure on the sail due to reflection and a small fraction that is absorbed. The absorbed energy heats the sail, which re-radiates that energy from the front and rear surfaces.
The momentum of a photon or an entire flux is given by p = E/c,^[11][12] where E is the photon or flux energy, p is the momentum, and c is the speed of light. Solar radiation pressure is calculated on an irradiance (solar constant) value of 1361 W/m² at 1 AU (Earth-Sun distance), as revised in 2011:^[13]

perfect absorbance: F = 4.54 μN per square metre (4.54 μPa)
perfect reflectance: F = 9.08 μN per square metre (9.08 μPa)  (normal to surface)

A perfect sail is flat and has 100% specular reflection. An actual sail will have an overall efficiency of about 90%, about 8.17 μN/m²,^[14] due to curvature (billow), wrinkles, absorbance, re-radiation from front and back, non-specular effects, and other factors.

Force on a sail results from reflecting the photon flux

The force on a sail and the actual acceleration of the craft vary by the inverse square of distance from the Sun (unless close to the Sun^[15]), and by the square of the cosine of the angle between the sail force vector and the radial from the Sun, so

F = F₀ cos² θ / R² (ideal sail)

where R is distance from the Sun in AU. An actual square sail can be modeled as:

F = F₀ (0.349 + 0.662 cos 2θ − 0.011 cos 4θ) / R²

Note that the force and acceleration approach zero generally around θ = 60° rather than 90° as one might expect with an ideal sail.^[16]

Solar wind, the flux of charged particles blown out from the Sun, exerts a nominal dynamic pressure of about 3 to 4 nPa, three orders of magnitude less than solar radiation pressure on a reflective sail.^[17]

Sail parameters

Sail loading (areal density) is an important parameter, which is the total mass divided by the sail area, expressed in g/m². It is represented by the Greek letter σ.

A sail craft has a characteristic acceleration, a_c, which it would experience at 1 AU when facing the Sun. Using the value from above of 9.08 μN per square metre of radiation pressure at 1 AU, a_c is related to areal density by:

a_c = 9.08(efficiency) / σ mm/s²

Assuming 90% efficiency, a_c = 8.17 / σ mm/s²

The lightness number, λ, is the dimensionless ratio of maximum vehicle acceleration divided by the Sun's local gravity. Using the values at 1 AU:

λ = a_c / 5.93

The lightness number is also independent of distance from the Sun because both gravity and light pressure fall off as the inverse square of the distance from the Sun. Therefore, this number defines the types of orbit maneuvers that are possible for a given vessel.

The table presents some example values. Payloads are not included. The first two are from the detailed design effort at JPL in the 1970s. The third, the lattice sailer, might represent about the best possible performance level.^[2] The dimensions for square and lattice sails are edges. The dimension for heliogyro is blade tip to blade tip.

Type	σ (g/m2)	ac (mm/s2)	λ	Size (km)
Square sail	5.27	1.56	0.26	0.820
Heliogyro	6.39	1.29	0.22	15
Lattice sailer	0.07	117	20	0.840

Attitude control

An active attitude control system (ACS) is essential for a sail craft to achieve and maintain a desired orientation. The required sail orientation changes slowly (often less than 1 degree per day) in interplanetary space, but much more rapidly in a planetary orbit. The ACS must be capable of meeting these orientation requirements.

Attitude control is achieved by a relative shift between the craft's center of pressure and its center of mass. This can be achieved with control vanes, movement of individual sails, movement of a control mass, or altering reflectivity.

Holding a constant attitude requires that the ACS maintain a net torque of zero on the craft. The total force and torque on a sail, or set of sails, is not constant along a trajectory. The force changes with solar distance and sail angle, which changes the billow in the sail and deflects some elements of the supporting structure, resulting in changes in the sail force and torque.

Sail temperature also changes with solar distance and sail angle, which changes sail dimensions. The radiant heat from the sail changes the temperature of the supporting structure. Both factors affect total force and torque.

To hold the desired attitude the ACS must compensate for all of these changes.^[18]

Constraints

In Earth orbit, solar pressure and drag pressure are typically equal at an altitude of about 800 km, which means that a sail craft would have to operate above that altitude. Sail craft must operate in orbits where their turn rates are compatible with the orbits, which is generally a concern only for spinning disk configurations.

Sail operating temperatures are a function of solar distance, sail angle, reflectivity, and front and back emissivities. A sail can be used only where its temperature is kept within its material limits. Generally, a sail can be used rather close to the Sun, around 0.25 AU, or even closer if carefully designed for those conditions.^[2]

Applications

Potential applications for sail craft range throughout the Solar System, from near the Sun to the comet clouds beyond Neptune. The craft can make outbound voyages to deliver loads or to take up station keeping at the destination. They can be used to haul cargo and possibly also used for human travel.^[2]

Inner planets

For trips within the inner Solar System, they can deliver loads and then return to Earth for subsequent voyages, operating as an interplanetary shuttle. For Mars in particular, the craft could provide economical means of routinely supplying operations on the planet.

Solar sail craft can approach the Sun to deliver observation payloads or to take up station keeping orbits. They can operate at 0.25 AU or closer. They can reach high orbital inclinations, including polar.

Solar sails can travel to and from all of the inner planets. Trips to Mercury and Venus are for rendezvous and orbit entry for the payload. Trips to Mars could be either for rendezvous or swing-by with release of the payload for aerodynamic braking.^[2]

Outer planets

Minimum transfer times to the outer planets benefit from using an indirect transfer (solar swing-by). However, this method results in high arrival speeds. Slower transfers have lower arrival speeds.

The minimum transfer time to Jupiter for a_c of 1 mm/s² with no departure velocity relative to Earth is 2 years when using an indirect transfer (solar swing-by). The arrival speed (V_∞) is close to 17 km/s. For Saturn, the minimum trip time is 3.3 years, with an arrival speed of nearly 19 km/s.^[2]

Minimum times to the outer planets (a_c = 1 mm/s²)

	Jupiter	Saturn	Uranus	Neptune
Time, yr	2.0	3.3	5.8	8.5
Speed, km/s	17	19	20	20

Oort Cloud / Sun's inner gravity focus

The Sun's inner gravity focus is a point, at about 550 AU in the inner Oort cloud, at which light from distant objects is focused by gravity as it passes the Sun. This is thus an ideal distance from which to observe the region of deep space on the other side of the Sun.^[19]

A small team had initially proposed a beryllium inflated sail that would go down to 0.05 AU from the Sun in order to get an acceleration peaking at 36.4 m/s², reaching a speed of 0.00264c (about 950 km/s) in less than a day. Such proximity to the Sun could prove to be impractical in the near term due to the structural degradation of beryllium at high temperatures, diffusion of Hydrogen at high temperatures as well as an electrostatic gradient, generated by the ionization of beryllium due to the solar wind, posing a burst risk; thus a revised perihelion of 0.1 AU was proposed to reduce the aforementioned temperature and solar flux exposure.^[20] Such a sail would take "Two and a half years to reach the heliopause, six and a half years to get to the Sun’s inner gravitational focus. with arrival at the inner Oort Cloud in no more than thirty years."^[19] "Such a mission could perform useful astrophysical observations en route, explore gravitational focusing techniques, and image Oort Cloud objects while exploring particles and fields in that region that are of galactic rather than solar origin."

Satellites

Robert L. Forward pointed out that a solar sail could be used to modify the orbit of a satellite around the Earth. In the limit, a sail could be used to "hover" a satellite above one pole of the Earth. Spacecraft fitted with solar sails could also be placed in close orbits about the Sun that are stationary with respect to either the Sun or the Earth, a type of satellite named by Forward a statite. This is possible because the propulsion provided by the sail offsets the gravitational potential of the Sun. Such an orbit could be useful for studying the properties of the Sun over long durations.^{[citation needed]}
Likewise a solar sail-equipped spacecraft could also remain on station nearly above the polar terminator of a planet such as the Earth by tilting the sail at the appropriate angle needed to just counteract the planet's gravity.^{[citation needed]}

In his book The Case for Mars, Robert Zubrin points out that the reflected sunlight from a large statite placed near the polar terminator of the planet Mars could be focused on one of the Martian polar ice caps to significantly warm the planet's atmosphere. Such a statite could be made from asteroid material.^{[citation needed]}

Trajectory corrections

The MESSENGER probe orbiting Mercury, used light pressure on its solar panels to perform fine trajectory corrections on the way to Mercury.^[21] By changing the angle of the solar panels relative to the Sun, the amount of solar radiation pressure was varied to adjust the spacecraft trajectory more delicately than possible with thrusters. Minor errors are greatly amplified by gravity assist maneuvers, so using radiation pressure to make very small corrections saved large amounts of propellant.

Interstellar flight

In the 1970s, Robert Forward proposed two beam-powered propulsion schemes using either lasers or masers to push giant sails to a significant fraction of the speed of light.^[22]

In The Flight of the Dragonfly, Forward described a light sail propelled by super lasers. As the starship neared its destination, the outer portion of the sail would detach. The outer sail would then refocus and reflect the lasers back onto a smaller, inner sail. This would provide braking thrust to stop the ship in the destination star system.

Both methods pose monumental engineering challenges. The lasers would have to operate for years continuously at gigawatt strength. Forward's solution to this requires enormous solar panel arrays to be built at or near the planet Mercury. A planet-sized mirror or fresnel lens would be needed several dozen astronomical units from the Sun to keep the lasers focused on the sail. The giant braking sail would have to act as a precision mirror to focus the braking beam onto the inner "deceleration" sail.

A potentially easier approach would be to use a maser to drive a "solar sail" composed of a mesh of wires with the same spacing as the wavelength of the microwaves, since the manipulation of microwave radiation is somewhat easier than the manipulation of visible light. The hypothetical "Starwisp" interstellar probe design^[23][24] would use microwaves, rather than visible light, to push it. Masers spread out more rapidly than optical lasers owing to their longer wavelength, and so would not have as long an effective range.

Masers could also be used to power a painted solar sail, a conventional sail coated with a layer of chemicals designed to evaporate when struck by microwave radiation.^[25] The momentum generated by this evaporation could significantly increase the thrust generated by solar sails, as a form of lightweight ablative laser propulsion.

To further focus the energy on a distant solar sail, designs have considered the use of a large zone plate. This would be placed at a location between the laser or maser and the spacecraft.^[22]

Another more physically realistic approach would be to use the light from the Sun to accelerate.^[26] The ship would first drop into an orbit making a close pass to the Sun, to maximize the solar energy input on the sail, then the ship would begin to accelerate away from the system using the light from the Sun to keep accelerating. Acceleration will drop approximately as the inverse square of the distance from the Sun, and beyond some distance, the ship would no longer receive enough light to accelerate it significantly, but would maintain its course due to inertia. When nearing the target star, the ship could turn its sails toward it and begin to use the outward acceleration to decelerate. Additional forward and reverse thrust could be achieved with more conventional means of propulsion such as rockets.

Similar solar sailing launch and capture were suggested for directed panspermia to expand life in other solar system. Velocities of 0.0005 c could be obtained by solar sails carrying 10 kg payloads, using thin solar sail vehicles with effective areal densities of 0.1 g/m² with thin sails of 0.1 µm thickness and sizes on the order of one square kilometer. Alternatively, swarms of 1 mm capsules can be launched on solar sails with radii of 42 cm, each carrying 10,000 capsules of a hundred million extremophile microorganisms to seed life in diverse target environments.^[27][28]

Deorbiting artificial satellites

Small solar sails have been proposed to accelerate the deorbiting of small artificial satellites from Earth orbits. Satellites in low Earth orbit can use a combination of solar pressure on the sail and increased atmospheric drag to accelerate satellite reentry.^[29]

Sail configurations

NASA illustration of the unlit side of a half-kilometre solar sail, showing the struts stretching the sail.

An artist's depiction of a Cosmos 1-type spaceship in orbit

IKAROS, launched in 2010, was the first practical solar sail vehicle. As of 2015, it was still under thrust, proving the practicality of a solar sail for long-duration missions.^[30] It is spin-deployed, with tip-masses in the corners of its square sail. The sail is made of thin polyimide film, with evaporated aluminium on it. It steers with electrically-controlled liquid crystal panels. The sail slowly spins, and these panels turn on and off to control the attitude of the vehicle. When on, they diffuse light, reducing the momentum transfer to that part of the sail. When off, the sail reflects more light, transferring more momentum. In that way, they turn the sail.^[31] Thin-film solar cells are also integrated into the sail, powering the spacecraft. The design is very reliable, because spin deployment, which is preferable for large sails, simplified the mechanisms to unfold the sail and the LCD panels have no moving parts.

Parachutes have very low mass, but a parachute is not a workable configuration for a solar sail. Analysis shows that a parachute configuration would collapse from the forces exerted by shroud lines, since radiation pressure does not behave like aerodynamic pressure, and would not act to keep the parachute open.^[32]

Eric Drexler^[33] proposed very high thrust-to-mass solar sails, and made prototypes of the sail material. His sail would use panels of thin aluminium film (30 to 100 nanometres thick) supported by a tensile structure. The sail would rotate and would have to be continually under thrust. He made and handled samples of the film in the laboratory, but the material was too delicate to survive folding, launch, and deployment. The design planned to rely on space-based production of the film panels, joining them to a deploy-able tension structure. Sails in this class would offer high area per unit mass and hence accelerations up to "fifty times higher" than designs based on deploy-able plastic films.^[33]

The highest thrust-to-mass designs for ground-assembled deploy-able structures are square sails with the masts and guy lines on the dark side of the sail. Usually there are four masts that spread the corners of the sail, and a mast in the center to hold guy-wires. One of the largest advantages is that there are no hot spots in the rigging from wrinkling or bagging, and the sail protects the structure from the Sun. This form can, therefore, go close to the Sun for maximum thrust. Most designs steer with small moving sails on the ends of the spars.^[34]

In the 1970s JPL studied many rotating blade and ring sails for a mission to rendezvous with Halley's Comet. The intention was to stiffen the structures using angular momentum, eliminating the need for struts, and saving mass. In all cases, surprisingly large amounts of tensile strength were needed to cope with dynamic loads. Weaker sails would ripple or oscillate when the sail's attitude changed, and the oscillations would add and cause structural failure. The difference in the thrust-to-mass ratio between practical designs was almost nil, and the static designs were easier to control.^[34]
JPL's reference design was called the "heliogyro". It had plastic-film blades deployed from rollers and held out by centrifugal forces as it rotated. The spacecraft's attitude and direction were to be completely controlled by changing the angle of the blades in various ways, similar to the cyclic and collective pitch of a helicopter. Although the design had no mass advantage over a square sail, it remained attractive because the method of deploying the sail was simpler than a strut-based design.^[34]

Heliogyro design is similar to the blades on a helicopter. The design is faster to manufacture due to lightweight centrifugal stiffening of sails. Also, they are highly efficient in cost and velocity because the blades are lightweight and long. Unlike the square and spinning disk designs, heliogyro is easier to deploy because the blades are compacted on a reel. The blades roll out when they are deploying after the ejection from the spacecraft. As the heliogyro travels through space the system spins around because of the centrifugal acceleration. Finally, payloads for the space flights are placed in the center of gravity to even out the distribution of weight to ensure stable flight.^[34]

JPL also investigated "ring sails" (Spinning Disk Sail in the above diagram), panels attached to the edge of a rotating spacecraft. The panels would have slight gaps, about one to five percent of the total area. Lines would connect the edge of one sail to the other. Masses in the middles of these lines would pull the sails taut against the coning caused by the radiation pressure. JPL researchers said that this might be an attractive sail design for large manned structures. The inner ring, in particular, might be made to have artificial gravity roughly equal to the gravity on the surface of Mars.^[34]

A solar sail can serve a dual function as a high-gain antenna.^[35] Designs differ, but most modify the metalization pattern to create a holographic monochromatic lens or mirror in the radio frequencies of interest, including visible light.^[35]

Electric solar wind sail

Pekka Janhunen from FMI has invented a type of solar sail called the electric solar wind sail.^[36] Mechanically it has little in common with the traditional solar sail design. The sails are replaced with straightened conducting tethers (wires) placed radially around the host ship. The wires are electrically charged to create an electric field around the wires. The electric field extends a few tens of metres into the plasma of the surrounding solar wind. The solar electrons are reflected by the electric field (like the photons on a traditional solar sail). The radius of the sail is from the electric field rather than the actual wire itself, making the sail lighter. The craft can also be steered by regulating the electric charge of the wires. A practical electric sail would have 50–100 straightened wires with a length of about 20 km each.^{[citation needed]}
Electric solar wind sails can adjust their electrostatic fields and sail attitudes.

Magnetic sail

A magnetic sail would also employ the solar wind. However, the magnetic field deflects the electrically charged particles in the wind. It uses wire loops, and runs a static current through them instead of applying a static voltage.^[37]
All these designs maneuver, though the mechanisms are different.

Magnetic sails bend the path of the charged protons that are in the solar wind. By changing the sails' attitudes, and the size of the magnetic fields, they can change the amount and direction of the thrust.

Sail making

Materials

The material developed for the Drexler solar sail was a thin aluminium film with a baseline thickness of 0.1 µm, to be fabricated by vapor deposition in a space-based system. Drexler used a similar process to prepare films on the ground. As anticipated, these films demonstrated adequate strength and robustness for handling in the laboratory and for use in space, but not for folding, launch, and deployment.

The most common material in current designs is aluminized 2 µm Kapton film. It resists the heat of a pass close to the Sun and still remains reasonably strong. The aluminium reflecting film is on the Sun side. The sails of Cosmos 1 were made of aluminized PET film (Mylar).

Research by Geoffrey Landis in 1998–1999, funded by the NASA Institute for Advanced Concepts, showed that various materials such as alumina for laser lightsails and carbon fiber for microwave pushed lightsails were superior sail materials to the previously standard aluminium or Kapton films.^[38]

In 2000, Energy Science Laboratories developed a new carbon fiber material that might be useful for solar sails.^[39] The material is over 200 times thicker than conventional solar sail designs, but it is so porous that it has the same mass. The rigidity and durability of this material could make solar sails that are significantly sturdier than plastic films. The material could self-deploy and should withstand higher temperatures.

There has been some theoretical speculation about using molecular manufacturing techniques to create advanced, strong, hyper-light sail material, based on nanotube mesh weaves, where the weave "spaces" are less than half the wavelength of light impinging on the sail. While such materials have so far only been produced in laboratory conditions, and the means for manufacturing such material on an industrial scale are not yet available, such materials could mass less than 0.1 g/m²,^[40] making them lighter than any current sail material by a factor of at least 30. For comparison, 5 micrometre thick Mylar sail material mass 7 g/m², aluminized Kapton films have a mass as much as 12 g/m²,^[34] and Energy Science Laboratories' new carbon fiber material masses 3 g/m².^[39]

The least dense metal is lithium, about 5 times less dense than aluminium. Fresh, unoxidized surfaces are reflective. At a thickness of 20 nm, lithium has an areal density of 0.011 g/m². A high-performance sail could be made of lithium alone at 20 nm (no emission layer). It would have to be fabricated in space and not used to approach the Sun. In the limit, a sail craft might be constructed with a total areal density of around 0.02 g/m², giving it a lightness number of 67 and a_c of about 400 mm/s². Magnesium and beryllium are also potential materials for high-performance sails. These 3 metals can be alloyed with each other and with aluminium.^[2]

Reflection and emissivity layers

Aluminium is the common choice for the reflection layer. It typically has a thickness of at least 20 nm, with a reflectivity of 0.88 to 0.90. Chromium is a good choice for the emission layer on the face away from the Sun. It can readily provide emissivity values of 0.63 to 0.73 for thicknesses from 5 to 20 nm on plastic film. Usable emissivity values are empirical because thin-film effects dominate; bulk emissivity values do not hold up in these cases because material thickness is much thinner than the emitted wavelengths.^[41]

Fabrication

Sails are fabricated on Earth on long tables where ribbons are unrolled and joined to create the sails. These sails are packed, launched, and unfurled in space.

In the future, fabrication could take place in orbit inside large frames that support the sail. This would result in lower mass sails and elimination of the risk of deployment failure.

Operations

A solar sail can spiral inward or outward by setting the sail angle

Changing orbits

Sailing operations are simplest in interplanetary orbits, where attitude changes are done at low rates. For outward bound trajectories, the sail force vector is oriented forward of the Sun line, which increases orbital energy and angular momentum, resulting in the craft moving farther from the Sun. For inward trajectories, the sail force vector is oriented behind the Sun line, which decreases orbital energy and angular momentum, resulting in the craft moving in toward the Sun. It is worth noting that only the Sun's gravity pulls the craft toward the Sun—there is no analog to a sailboat's tacking to windward. To change orbital inclination, the force vector is turned out of the plane of the velocity vector.

In orbits around planets or other bodies, the sail is oriented so that its force vector has a component along the velocity vector, either in the direction of motion for an outward spiral, or against the direction of motion for an inward spiral.

Trajectory optimizations can often require intervals of reduced or zero thrust. This can be achieved by rolling the craft around the Sun line with the sail set at an appropriate angle to reduce or remove the thrust.^[42]

Swing-by Maneuvers

A close solar passage can be used to increase a craft's energy. The increased radiation pressure combines with the efficacy of being deep in the Sun's gravity well to substantially increase the energy for runs to the outer Solar System. The optimal approach to the Sun is done by increasing the orbital eccentricity while keeping the energy level as high as practical. The minimum approach distance is a function of sail angle, thermal properties of the sail and other structure, load effects on structure, and sail optical characteristics (reflectivity and emissivity). A close passage can result in substantial optical degradation. Required turn rates can increase substantially for a close passage. A sail craft arriving at a star can use a close passage to reduce energy, which also applies to a sail craft on a return trip from the outer Solar System.

A lunar swing-by can have important benefits for trajectories leaving from or arriving at Earth. This can reduce trip times, especially in cases where the sail is heavily loaded. A swing-by can also be used to obtain favorable departure or arrival directions relative to Earth.

A planetary swing-by could also be employed similar to what is done with coasting spacecraft, but good alignments might not exist due to the requirements for overall optimization of the trajectory.^[43]

Smart lines

A smart line could be a critical element of sailing operations. As with maritime ships, lines are essential for a wide range of uses. One difference is that some lines may be very long and need to be self-guiding. The lines could extend from and retract into the sail craft.

A maneuverable grappling device can be used at the end of a line to place or pick up payload containers, to secure a ship to a structure such as a station, to pick up samples from an asteroid or comet, or to engage in towing. The maneuvering unit is like a small spacecraft, with many of the same sensors and control systems. It could draw power from and communicate with the sail craft through the line. These operations could be done autonomously.

Lines a few hundred kilometers long may be used to move a ship from a space station to an orbit farther out where it could begin sailing.^[44]

Towing

Smart lines can enable towing operations by being able to attach to or release objects at the remote end of the line. Attached objects might be pulled in to the body of the sailer or remain at the end of the deployed line. Objects to be towed may have attachment points that allow multiple sail craft to engage in the towing. Towing operations can include deflecting large bodies that pose a hazard to Earth, bringing natural bodies to Earth or other sites for resource recovery, and transporting disabled spacecraft or other structures.

To tow or deflect a large body, poles can be inserted on the spin axis of the body. Sail craft can attach to the embedded poles using smart lines. Slip rings enable the craft to tow without the lines getting wrapped up as a result of rotation of the body.^[45][46]

Projects operating or completed

IKAROS 2010

The model of IKAROS at the 61st International Astronautical Congress in 2010

On 21 May 2010, Japan Aerospace Exploration Agency (Jaxa) launched the world's first interplanetary solar sail spacecraft "IKAROS" (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) to Venus.^[47]

Japan's JAXA successfully tested IKAROS in 2010. The goal was to deploy and control the sail and, for the first time, to determine the minute orbit perturbations caused by light pressure. Orbit determination was done by the nearby AKATSUKI probe from which IKAROS detached after both had been brought into a transfer orbit to Venus. The total effect over the six month' flight was 100 m/s.^[48]

Until 2010, no solar sails had been successfully used in space as primary propulsion systems. On 21 May 2010, the Japan Aerospace Exploration Agency (JAXA) launched the IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) spacecraft, which deployed a 200 m² polyimide experimental solar sail on June 10.^[49][50][51] In July, the next phase for the demonstration of acceleration by radiation began. On 9 July 2010, it was verified that IKAROS collected radiation from the Sun and began photon acceleration by the orbit determination of IKAROS by range-and-range-rate (RARR) that is newly calculated in addition to the data of the relativization accelerating speed of IKAROS between IKAROS and the Earth that has been taken since before the Doppler effect was utilized.^[52] The data showed that IKAROS appears to have been solar-sailing since 3 June when it deployed the sail.

IKAROS has a diagonal spinning square sail 14×14 m (196 m²) made of a 7.5-micrometre (0.0075 mm) thick sheet of polyimide. The polyimide sheet had a mass of about 10 grams per square metre. A thin-film solar array is embedded in the sail. Eight LCD panels are embedded in the sail, whose reflectance can be adjusted for attitude control.^[53][54] IKAROS spent six months traveling to Venus, and then began a three-year journey to the far side of the Sun.^[55]

Attitude (orientation) control

Both the Mariner 10 mission, which flew by the planets Mercury and Venus, and the MESSENGER mission to Mercury demonstrated the use of solar pressure as a method of attitude control in order to conserve attitude-control propellant.

Hayabusa also used solar pressure on its solar paddles as a method of attitude control to compensate for broken reaction wheels and chemical thruster.

MTSAT-1R (Multi-Functional Transport Satellite )'s solar sail counteracts the torque produced by sunlight pressure on the solar array. The trim tab on the solar array makes small adjustments to the torque balance.

Sail deployment tests

NASA has successfully tested deployment technologies on small scale sails in vacuum chambers.^[56]
On February 4, 1993, the Znamya 2, a 20-meter wide aluminized-mylar reflector, was successfully deployed from the Russian Mir space station. Although the deployment succeeded, propulsion was not demonstrated. A second test, Znamya 2.5, failed to deploy properly.

In 1999, a full-scale deployment of a solar sail was tested on the ground at DLR/ESA in Cologne.^[57]
On August 9, 2004, the Japanese ISAS successfully deployed two prototype solar sails from a sounding rocket. A clover-shaped sail was deployed at 122 km altitude and a fan-shaped sail was deployed at 169 km altitude. Both sails used 7.5-micrometer film. The experiment purely tested the deployment mechanisms, not propulsion.^[58]

Solar sail propulsion attempts

A photo of the experimental solar sail, NanoSail-D.

A joint private project between Planetary Society, Cosmos Studios and Russian Academy of Science made two sail testing attempts: in 2001 a suborbital prototype test failed because of rocket failure; and in June 21, 2005, Cosmos 1 launched from a submarine in the Barents Sea, but the Volna rocket failed, and the spacecraft failed to reach orbit. They intended to use the sail to gradually raise the spacecraft to a higher Earth orbit over a mission duration of one month.
A 15-meter-diameter solar sail (SSP, solar sail sub payload, soraseiru sabupeiro-do) was launched together with ASTRO-F on a M-V rocket on February 21, 2006, and made it to orbit. It deployed from the stage, but opened incompletely.^[59]

NanoSail-D 2010

A team from the NASA Marshall Space Flight Center (Marshall), along with a team from the NASA Ames Research Center, developed a solar sail mission called NanoSail-D, which was lost in a launch failure aboard a Falcon 1 rocket on 3 August 2008.^[60][61] The second backup version, NanoSail-D2, also sometimes called simply NanoSail-D,^[62] was launched with FASTSAT on a Minotaur IV on November 19, 2010, becoming NASA's first solar sail deployed in low earth orbit. The objectives of the mission were to test sail deployment technologies, and to gather data about the use of solar sails as a simple, "passive" means of de-orbiting dead satellites and space debris.^[63] The NanoSail-D structure was made of aluminium and plastic, with the spacecraft massing less than 10 pounds (4.5 kg). The sail has about 100 square feet (9.3 m²) of light-catching surface. After some initial problems with deployment, the solar sail was deployed and over the course of its 240-day mission reportedly produced a "wealth of data" concerning the use of solar sails as passive deorbit devices.^[64]NASA launched the second NanoSail-D unit stowed inside the FASTSAT satellite on the Minotaur IV on November 19, 2010. The ejection date from the FASTSAT microsatellite was planned for December 6, 2010, but deployment only occurred on January 20, 2011.^[65]

LightSail-A

On Carl Sagan's 75th birthday (November 9, 2009) the Planetary Society announced plans^[66] to make three further attempts, dubbed LightSail-1, -2, and -3.^[67] The new design will use a 32 m² Mylar sail, deployed in four triangular segments like NanoSail-D.^[67] The launch configuration is a 3U CubeSat format, and as of 2015, it is scheduled as a secondary payload for a 2016 launch on the first SpaceX Falcon Heavy launch.^[68] The Planetary Society of the United States initiated a short test of an artificial satellite "LightSail-A" that launched on 20 May 2015.^[69] The purpose of the test is to allow a full checkout of the satellite's systems in advance of the main 2016 mission, LightSail-1.

Projects in development or proposed

Despite the losses of Cosmos 1 and NanoSail-D (which were due to failure of their launchers), scientists and engineers around the world remain encouraged and continue to work on solar sails. While most direct applications created so far intend to use the sails as inexpensive modes of cargo transport, some scientists are investigating the possibility of using solar sails as a means of transporting humans. This goal is strongly related to the management of very large (i.e. well above 1 km²) surfaces in space and the sail making advancements. Manned space flight utilizing solar sails is still in the development state of infancy.

Sunjammer 2015

A technology demonstration sail craft, dubbed Sunjammer, was in development with the intent to prove the viability and value of sailing technology.^[70] Sunjammer had a square sail, 124 feet (38 meters) wide on each side (total area 13,000 sq ft or 1,208 sq m). It would have traveled from the Sun-Earth L₁ Lagrangian point 900,000 miles from Earth (1.5 million km) to a distance of 1,864,114 miles (3 million kilometers).^[71] The demonstration was expected to launch on a Falcon 9 in January 2015.^[72] It would have been a secondary payload, released after the placement of the DSCOVR climate satellite at the L1 point.^[72] Citing a lack of confidence in its contractor's ability to deliver, the mission was cancelled in October 2014.^[73]

Gossamer deorbit sail

As of December 2013^[update], the European Space Agency (ESA) has a proposed deorbit sail, named "Gossamer", that would be intended to be used to accelerate the deorbiting of small (less than 700 kilograms (1,500 lb)) artificial satellites from low-Earth orbits. The launch mass is 2 kilograms (4.4 lb) with a launch volume of only 15×15×25 centimetres (0.49×0.49×0.82 ft). Once deployed, the sail would expand to 5 by 5 metres (16 ft × 16 ft) and would use a combination of solar pressure on the sail and increased atmospheric drag to accelerate satellite reentry.^[29]

NEA Scout

NEA Scout concept: a controllable CubeSat solar sail spacecraft

The Near-Earth Asteroid Scout (NEA Scout) is a proposed mission concept by NASA to develop a controllable low-cost CubeSat solar sail spacecraft capable of encountering near-Earth asteroids (NEA). Four 7-m booms would deploy, unfurling the 83 m² aluminized polyimide solar sail.^[74][75][76]

In science fiction

The earliest reference to solar sailing was in Jules Verne's 1865 novel From the Earth to the Moon, coming only a year after Maxwell's equations were published. The next known publication came more than 20 years later when Georges Le Faure and Henri De Graffigny published a four-volume science fiction novel in 1889, The Extraordinary Adventures of a Russian Scientist, which included a spacecraft propelled by solar pressure. B. Krasnogorskii published On the Waves of the Ether in 1913. In his story backed by technical calculations, a small, bullet-shaped capsule is surrounded by a circular mirror 35 meters in diameter. It travels through space by means of solar pressure on the mirror.

One of the earliest American stories about light sails is "The Lady Who Sailed the Soul" by Cordwainer Smith, which was published in 1960. In it, a tragedy results from the slowness of interstellar travel by this method. Another example is the 1962 story "Gateway to Strangeness" (also known as "Sail 25") by Jack Vance, in which the outward direction of propulsion poses a life-threatening dilemma. Also in early 20th century literature, Pierre Boulle's Planet of the Apes novel starts with a couple floating in space on a ship propelled and maneuvered by light sails. In Larry Niven and Jerry Pournelle's The Mote in God's Eye, a sail is used as a brake and a weapon.

Both Arthur C. Clarke and Poul Anderson , independently, but simultaneously, published distinct stories titled "Sunjammer" in March 1964. Clarke's depicted a "yacht race" between solar sail spacecraft, while Anderson, writing as Winston P. Sanders, dipicts a maintenance crew, servicing space-freighters powered by light sails. Clarke published in the March 1964 issue of Boys' Life, while Anderson published in the April 1964 (on sale March 12, 1964) issue of Analog Science Fiction / Science Fact.

Interstellar travel by using light sails are also an integral part of Buzz Aldrin and John Barnes science fiction novel "Encounter with Tiber", where the alien race 'the tiberians' use a solar sail powered first by a close flybys with Alpha Centauri A and B followed by laser boosting to travel to Earth some 9000 years ago. Their spaceship decelerate by detaching part of the light sail as well as using magnetic braking in the solar wind.

In "The Flight of the Dragonfly", Robert Forward (who also proposed the microwave-pushed Starwisp design) described an interstellar journey using a light driven propulsion system, wherein a part of the sail was broken off and used as a reflector to slow the main spacecraft as it approached its destination. Forward's ideas were developed further in Charles Stross's novel Accelerando.^[77] In the 1982 film Tron, a "Solar Sailer" was an inner spacecraft with butterfly like sails moved along focused beam of light. The 1983 episode "Enlightenment" of Doctor Who featured sailing ships in space that used solar wind to fly. In the episode "Explorers" of Star Trek: Deep Space Nine that aired in 1995, a reconstructed, "ancient" Bajoran "light ship" was featured. It was designed to use solar wind to fly out of a Solar System with no engine. In the film Star Wars Episode II: Attack of the Clones one is used by Count Dooku to propel himself across space. A solar sail was also used in James Cameron's Avatar. In the Disney film Treasure Planet, solar sails are used literally as sails for interstellar travel as well as serving for the photovoltaic gathering of energy for the jet propulsion of a steampunk-styled masted sailing ship capable of traveling through space.

Perth electrical engineer’s discovery will change climate change debate

October 3, 2015 12:00pm

MIRANDA DEVINE,  PerthNow

 

Original link:   http://www.ntnews.com.au/lifestyle/miranda-devine-perth-electrical-engineers-discovery-will-change-climate-change-debate/story-fnk0b1ks-1227555674611

Dr David Evans has unpacked the architecture of the basic climate model which underpins all climate science. Picture: Thinkstock

A MATHEMATICAL discovery by Perth-based electrical engineer Dr David Evans may change everything about the climate debate, on the eve of the UN climate change conference in Paris next month.

A former climate modeller for the Government’s Australian Greenhouse Office, with six degrees in applied mathematics, Dr Evans has unpacked the architecture of the basic climate model which underpins all climate science.

He has found that, while the underlying physics of the model is correct, it had been applied incorrectly.

He has fixed two errors and the new corrected model finds the climate’s sensitivity to carbon dioxide (CO2) is much lower than was thought.

Miranda Devine. Picture: Peter Brew-Bevan

It turns out the UN’s Intergovernmental Panel on Climate Change has over-estimated future global warming by as much as 10 times, he says.

“Yes, CO2 has an effect, but it’s about a fifth or tenth of what the IPCC says it is. CO2 is not driving the climate; it caused less than 20 per cent of the global warming in the last few decades”.

Dr Evans says his discovery “ought to change the world”.

“But the political obstacles are massive,” he said.

His discovery explains why none of the climate models used by the IPCC reflect the evidence of recorded temperatures. The models have failed to predict the pause in global warming which has been going on for 18 years and counting.

“The model architecture was wrong,” he says. “Carbon dioxide causes only minor warming. The climate is largely driven by factors outside our control.”

There is another problem with the original climate model, which has been around since 1896.
While climate scientists have been predicting since the 1990s that changes in temperature would follow changes in carbon dioxide, the records over the past half million years show that not to be the case.

So, the new improved climate model shows CO2 is not the culprit in recent global warming. But what is?

Dr Evans has a theory: solar activity. What he calls “albedo modulation”, the waxing and waning of reflected radiation from the Sun, is the likely cause of global warming.

He predicts global temperatures, which have plateaued, will begin to cool significantly, beginning between 2017 and 2021. The cooling will be about 0.3C in the 2020s. Some scientists have even forecast a mini ice age in the 2030s.

If Dr Evans is correct, then he has proven the theory on carbon dioxide wrong and blown a hole in climate alarmism. He will have explained why the doomsday predictions of climate scientists aren’t reflected in the actual temperatures.

Dr David Evans, who says climate model architecture is wrong, with wife Jo Nova, Picture: australianclimatemadness.com

“It took me years to figure this out, but finally there is a potential resolution between the insistence of the climate scientists that CO2 is a big problem, and the empirical evidence that it doesn’t have nearly as much effect as they say.”

Dr Evans is an expert in Fourier analysis and digital signal processing, with a PhD, and two Masters degrees from Stanford University in electrical engineering, a Bachelor of Engineering (for which he won the University medal), Bachelor of Science, and Masters in Applied Maths from the University of Sydney.

He has been summarising his results in a series of blog posts on his wife Jo Nova’s blog for climate sceptics.

He is about half way through his series, with blog post 8, “Applying the Stefan-Boltzmann Law to Earth”, published on Friday.

When it is completed his work will be published as two scientific papers. Both papers are undergoing peer review.

“It’s a new paradigm,” he says. “It has several new ideas for people to get used to.”

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